Hm, I know you already have a strong opinion about this, but I try anyway to make some points in the other direction. :)

Most importantly, I'm not a big friend of void-methods. In this case here, it would always force a user to jump the loop of declaring and initializing a variable, calling the mapping method, and going on working with the variable. The non-void alternative would allow users to do it in one line: `return myMapper.sourceMapToTargetMap(computeInternalResult(...), new ConcurrentHashMap<>());`

There are many people out there praising the functional-style, and we should support that.

Making the alternatives transparent for a user, I guess, is a matter of structuring the documentation - not all three variants need to show up in the front page's example :)

So, any chance you might give it some more thought? :)